This invention relates to an isotope filter device for optical pumping and finds application in frequency control and spectroscopic devices.
Optical pumping schemes for producing monochromatic pumping light (other than laser systems, which require elaborate frequency stabilization) suffer from low efficiency because the desired optical pumping frequency is one component of a plurality of hyperfine components with the plurality forming a background light signal which tends to mask the pumping light. The signal to noise ratio is very low, and the desired optical pumping light is greatly masked.
Prior art frequency control devices using such optical pumping include Rubidium frequency standards. These generally use some form of lamp and filter for optical pumping, and the optical pumping signal at the resonance frequency is limited to one or two percent due to background light.
An example of a prior art rubidium frequency standard is illustrated in the article Time Frequency and Physical Measurement by Hellwig, Evenson and Wineland appearing in the December, 1978 issue of Physics Today. FIG. 3 of that article illustrates the Rubidium atomic reasonator. That diagram is substantially reproduced in FIG. 1 of this patent application and shows a Rb lamp 10 excited by a rf lamp exciter 12 to product light which is supplied to a filter cell 14 carrying Rb85 and a buffer gas. The light emanating from lamp 10 filtered by filter cell 14 passes into a resonance cell assembly 16 formed by an absorption filter which comprises an absortion cell 18 containing Rb87 and a buffer gas placed in a cavity 20 which also is provided with a weak magnetic field, the C-field. The cavity, absorption cell and filter are magnetically shielded and the resulting resonance signal is observed at detector 22.
In operation, the apparatus of FIG. 1 utilizes a spectrum overlap between the emission lines of the Rb87 lamp and the absorpiton lines of the Rb85 filter cell 14 to produce an asymmetric transmitted spectrum capable of optical pumping in the resonance absorption cell 18. This light passes through the resonance cell 18 where it is preferentially absorbed by one of the ground states in the Rb87 atom. The result of this optical pumping is an imbalance in populations in steady state which is shown as a partial bleaching of the absorption coefficient in the resonance cell atoms providing enhanced transmission of light. When a microwave signal at the resonance frequency interrogates the cell, this bleaching action is interrupted because ground state populations are equalized by the resonant microwave field. The resulting optical signal (which is then used to control the microwave frequency) is typically only one or two percent of the total light signal. This is because although only one of the hyperfine components is needed for the depletion pumping of the ground state atoms in the resonance cell, all of the hyperfine components impinge on the detector to produce a large background signal which dilutes the resonance signal.